2-Acyl imidazoles

C-Acylation imidazoles in, 1, 463 ortiio-Acylation 2-alkylpyridines, 1, 436 Acylimines... 

Although acyl imidazoles can be replaced by phenyl esters in some cases," acyl imidazoles are more effecdve for the acyladon of nitroalkanes A combinadon of diethyl phosphorcyani-dates and triethylamime allows the direct C-acyladon of nitromethane by aromadc carboxylic... 

Bei der Herstellung von Aldehyden iiber die 1-Acyl-imidazole wird die Carbonsaure mit l-[Imidazolyl-(l)-carbonyI]-imidazol umgesetzt und das erhaltene Imidazol-Derivat ohne Isolierung zum Aldehyd reduziert5 ... 

In general, reduction of amides to alcohols is difficult. More commonly the amide is reduced to an amine. An exception uses LiH2NBH3 to give the alcohol. Reduction with sodium metal in propanol also gives the alcohol.Acyl imidazoles are also reduced to the corresponding alcohol with NaBH4 in aqueous HC1. °... 

Improved selective acylation of 4,6 4, 6 -di-0-benzylidene-a,a-trehalose was achieved in acetone by the following method, in which a gentle liberation of -acyl-imidazole occurs on the conversion of benzotriazolyl-iV-oxytris(dimethylamino)phos-phonium hexafluorophosphate with the carboxylic acid in the presence of triethylamine and addition of imidazole 1199 ... 

The acylation of the carbanions derived from nitroalkanes with acyl imidazoles or alkoxy-carbonylimidazoles takes place at the carbon atom to yield a-nitro ketones or a-nitro esters, respectively (Eq. 5.10).21 The lithium salts of nitroalkanes were isolated and used in THF or DMSO in the original procedure. Later, potassium salts generated in situ on treatment with r-BuOK in DMSO are reactive enough to give a-nitro ketones in good yield (Eq. 5.11).22... 

In summary, reactions of nitronates with acid anhydrides or acyl chlorides give the O-acylated products, and reactions with acyl imidazoles, phenyl esters, acyl nitriles, and enol-lactones gives the C-acylated products, (see Eq. 5.13).25 The C/O selectivity of nitronate acylation by RCOX is qualitatively correlated with strength (pKJ of the acid HX conjugated to the leaving group X .25... 

Bis(oxazolinyl)pyridine-Ce(IV) triflate complex 78 catalyzed the enantioselective 1,3-DC of acyclic nitrones with a, 3-unsaturated 2-acyl imidazoles. For example, C-phenyl 7V-benzyl nitrone reacted with 77 in the presence of 78 to give the adduct 79 with excellent diastereo-and enantioselectivity. Isoxazolidine 79 was then converted into P -hydroxy-P-amino acid derivatives by hydrogenation of the N-0 bond in the presence of Pd(OH)2/C and cleavage of the 2-acyl imidazole with MeOTf in MeCN <06OL3351>. 

Pyrimidin-1-oxide lagern sich in allerdings nur maBigen Ausbeuten photochemisch in 4(5)-Acyl-imidazole um435,436. Die beste Ausbeute wurde mit 35% bei dcr Herstellung von 4(5)-Methoxycarbonyl-imidazol aus 5-Methoxy-pyrimidin-l-oxid erzielt436. 

Alkyl-imidazole sind sehr stabile Verbindungen. Dagegen werden 1-Acyl-imidazole bereits durch Erwarmen in Wasser hydrolysiert536. Die auBerst einfache nukleophile Substituierbarkeit des Imidazol-Restes von 1-Acyl-imidazolen wird haufig zur Aktivierung der Carbonsaure-Funktion genutzt (Bd. XII/2, S. 270, 382, 408 Bd. E3, S. 462 Bd. E4, S. 180 Bd. E5/2, S. 1164, 1247, 1249). 

Acyl-imidazole reagieren bereits unter milden Bedingungen mit zahlreichen Nukleophilen unter Austausch des Imidazol-Restes. Deshalb werden sie oft zur Aktivierung der Carbonsaure-Funktion verwendet (s. S.99f., 159f). 

Acyl-imidazole konnen auch zur Wasser-Abspaltung aus organischen Hydroxy-Verbindun-gen genutzt werden. So wurden zahlreiche Aldoxime mit 1-Trifluoracetyl-imidazol zu den ent-sprechenden Carbonsaure-nitrilen dehydratisiert1223. 

Benzophenon kann photochemisch in einer [2 + 2]-Cycloaddition an 1-Acyl-imidazole addiert werden (34-51 %)1248-1249 ... 

They are easily prepared from Grignard reagents, magnesium ethoxide or with the complex magnesium chloride-trialkylamine (see Section II where the generation of these reagents is discussed). The acylation of anions derived from malonates can be achieved with acyl chlorides, acyl imidazoles, aUcoxycarbonylimidazoles or mixed anhydrides. 

Carbonyldiimidazole does certainly stand out as one of the most remarkable reagents for peptide coupling. Carboxylic acids react readily with this reagent to yield acyl imidazoles which in turn couple with the amino group of a peptide to produce a new acyl-nitrogen linkage with very little racemization. Reaction of the acylimidazolide with other nucleophiles can lead to a variety of carbonyl derivatives (62AG(E)35l). 

Page et al. (see [298] and references therein) have shown that generally excellent stereocontrol in organic reactions can be obtained by using DITOX (1,3-dithiane-l-oxide) derivatives as chiral auxiliaries. The one-pot stereo-controlled cycloalkanone synthesis given here outlines some aspects of the chemistry worked out for efficient acylation-alkylations steps. Of note are the use of N-acyl imidazoles under mixed base (sodium hexamethyldisilazide/n-butyllithium) conditions to yield the lithium enolates of 2-acyl-l,3-dithiane-l-oxides) and the sequential alkylation-cyclization of the latter (steps (iv) and (v)). 

Acyl imidazoles take part in a silyl acyloin reaction to give the corresponding silyl enol ethers in moderate yields. A possible mechanism is outlined in Scheme 2092. The silyl enol ethers could be hydrolysed to acyl silanes by treatment with acid. 

Here it is assumed that the hydrogen, which is eliminated with the alcohol moiety R OH, stems from the imidazol ring, so that the acyl-imidazol XII appears as intermediate. However, other sources of this hydrogen atom have to be considered (i.e., either other components of the active center or the solvent). If the hydrogen in question originates from... 

In addition to moving the sulfonyl chloride earlier in the synthesis, the commercial route also uses an acyl imidazole to form a key amide bond (13.77 — 13.79) instead of an acid chloride (13.67 — 13.70, Scheme 13.12). Carbonyldiimidazole (13.76) is an expensive reagent, but the use of acyl imidazole 13.77 gives many advantages. The yields of the amide-forming reaction are higher, and solvent requirements are greatly simplified. The overall... 

Enantioselective additions of a,f)-unsaturated 2-acyl imidazoles, catalyzed by bis(oxazolinyl)pyridine-scandium(III)triflate complex, were used for the asymmetric synthesis of 3-substituted indoles. The complex 114 was one of the most promising catalysts. The choice of acetonitrile as the solvent and the use of 4 A molecular sieves were also found to be advantageous. The 2-acyl imidazole residue in the alkylation products of u,(i-unsaturated 2-acyl imidazoles could be transformed into synthetically useful amides, esters, carboxylic acid, ketones, and aldehydes (Scheme 32) [105]. Moreover, the catalyst 114 produced both the intramolecular indole alkylation and the 2-substituted indoles in good yield and enantioselectivity (Scheme 33) [106]. The complex... 

Enantioselective Michael additions of pyrroles to a,(3-unsaluralcd 2-acyl imidazoles were accomplished by Sc(III)triflate-bis(oxazolinyl)pyridine com-... 

---